1. Field of the Invention
This invention relates to a method for effecting image reduction processing of image information obtained from a given original image by using an image reading means such as an image scanner, and more particularly to a method for subjecting a halftone image such as a photograph to image reduction processing in accordance with pixel data at criterion points in a prescribed matrix pattern, thereby to output a resultant reduced image to a monitor display of an image processing system, a printer of a word processor or the like with high fidelity of image to the original image.
2. Description of the Prior Art
In a case of subjecting a given image to image processing by utilization of an image processing device such as a word processor and a facsimile, there are frequent occasions when a resultant image which is electrically read by using an image reading means such as an image scanner is reproduced on a monitor display unit or printed in hard copy form at a reduced scale. When an given original image of 100 mm in width is read and quantized by use of an ordinary image scanner having a resolution of 8 dots/mm, a resultant image thus obtained and represented in terms of digital bits is composed of picture elements of as many as 800 dots in total in the width direction. However, an ordinary monitor display such as a CRT which is widely applied to general purpose computers for effecting image processing has generally a display field of a resolution of 640.times.400 picture elements. This means that the read image obtained by the image scanner cannot be completely displayed on such a monitor display. Thus, it is necessary to reduce a read image to be displayed on the monitor display. Therefore, an image reduction processing has been conventionally performed simply by thinning out, for example, every other pixel to reduce the read image to a scale of one-half.
A dither image-processing method are available for subjecting a halftone image to image digitization. This dither image-processing method will be explained in brief with reference to the typical dither pattern model shown in FIG. 1(A). First, a given original image Io to be subjected to image processing is defined in a matrix and the brightness at each picture element of the matrix of the original image is measured to obtain a brightness matrix pattern Mb. In the illustrated dither pattern model, the brightness is expressed in 16 gradient levels as illustrated in FIG. 1(B). Next, the brightness at each picture element of the brightness matrix pattern Mb is compared with threshold value prescribed in each pixel of a predetermined dither pattern Pd. When the level value of brightness at one picture element of the brightness matrix pattern Mb is less than the threshold value of the corresponding pixel of the dither pattern Pd, the picture element is determined to be "black". Thus, an intermediate dither image Id is obtained and outputted to a monitor display or printer. This dither image processing method has been generally applied to image processing because it can reproduce the given halftone image with relatively high fidelity of image to the original.
In a case where the intermediate dither image Id obtained as a result of the dither image processing noted above is reduced to 1/2, there have been so far used an image reducing method as shown in FIGS. 2(A) to 2(C). According to the conventional method, in its own way, the intermediate dither image Id can be reduced to one-half by reducing every 2.times.2 matrix to one picture element. In the image reducing method shown in FIG. 2(A), the brightness feature in every picture element of a resultant reduced image Ir to be outputted to a monitor display or the like is determined on the basis of the brightness of one pixel at a specific location in a prescribed unit matrix. Namely, in the first model illustrated in FIG. 2(A), the upper and left picture elements (pixels a) in every unit matrix with 2.times.2 pixels are simply applied to the respective picture elements a' in the reduced image Ir. In the second model illustrated in FIG. 2(B), when the unit matrix with 2.times.2 pixels in the intermediate dither image Id has two or more black picture elements, the corresponding picture element in the reduced image Ir is determined as a "black" picture element Conversely, when the unit matrix with 2.times.2 pixels of the intermediate dither image Id has two or more white picture elements, the corresponding picture element in the reduced image Ir may be determined as a "white" picture element as illustrated in FIG. 2(C). However, as is apparent from the resulting patterns of the reduced images Ir, all the reduced images are wholly different in visual impressions from the given original image Io. Thus, the conventional image reducing methods could not process a halftone image such as a photograph with good reproductivity of image and were not practical use.